1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more specifically, to an image forming apparatus based on an electrophotographic system or an electrostatic printing system, such as a copier or a laser beam printer.
2. Description of the Related Art
An image forming apparatus such as a copier or an LBP (Laser Beam Printer) based on an electrophotographic system has an electrophotographic photosensitive member (hereinafter referred to as a “photosensitive member”) which is shaped like a rotating drum or belt and which operates as an image carrier, charging means for charging the photosensitive member to a predetermined potential, latent image forming means for forming an electrostatic latent image by exposing the photosensitive member charged by the charging means, and image forming process means for developing the electrostatic latent image.
The image forming apparatus forms a toner image on the photosensitive member which is transferable and which corresponds to image information. Then, transfer means transfers the toner image from the photosensitive member to a print material. Moreover, the print material to which the toner image has been transferred is introduced into fixing means to thermally fix the toner image to a surface of the print material as a permanently fixed image. The print material is then outputted as image formed matter (a copy or a print). After the toner image has been transferred to the print material, residual attached contaminants such as transfer residual toner or paper dusts remaining on the surface of the photosensitive member are removed (photosensitive member cleaning) so that the photosensitive member can be repeatedly used for an image forming process.
In a transfer section corresponding to a nip portion that is in pressure contact with the photosensitive member, transfer means is frequently used which uses a contact rotation type transfer member, what is called a transfer roller, which electrostatically transfers the toner image from the photosensitive member to the print material while sandwiching and conveying the print material. The transfer roller is used because it has the advantages of serving to simplify a conveying path for the print material and allowing the print material to be stably conveyed. The transfer section applies a plus bias from the transfer means which bias is the reverse of a toner charging polarity (for example, a minus charging characteristic), to the photosensitive member via the print material. Thereby, an electric field is formed to transfer the toner image from the photosensitive member to a print material.
An image forming apparatus is known which has a relatively long distance between the transfer section and the fixing section, located downstream in a conveying direction, and in which conveying auxiliary means including an elastomer roller, an elastomer belt, and the like is disposed between the transfer section and the fixing section in order to convey a print material that is shorter than the distance between the transfer section and the fixing section.
FIG. 1 shows the structure of a conveying auxiliary device in a conventional image forming apparatus. A conveying auxiliary device 16 conveys a print material P discharged from between a photosensitive drum 1 and a transfer roller 5, to the fixing device. The conveying auxiliary device 16 has an elastomer conveying belt 16a that is an endless belt having a width of about 5 to 100 mm. The elastomer conveying belt 16a has steps 16b of about 0.1 to 1 mm on its surface at a pitch of about 1 to 10 mm. Normally, the elastomer conveying belt 16a is tensioned by a plurality of shafts 23a and 23b provided on the side of a nonprinted surface of the print material. Particularly if the print material is shorter than the distance between the transfer roller and the fixing device, rotation of the driving shaft 23a in the conveying direction assists the conveyance of the print material already passed through a transfer nip N to the fixing device so that a trailing end of the print material in the conveying direction is pushed by the steps 16b. 
An inlet side of the transfer nip N is defined as Na. An outlet side of the transfer nip N is defined as Nb. A central position of the transfer nip N is defined as No. If the conveying auxiliary device 16 is located too high, an unfixed image on a printed surface may be disturbed. Accordingly, considerations are given so that the conveying auxiliary device 16 is located somewhat along the conveying surface between the transfer roller and the fixing device from the transfer nip outlet Nb.
FIG. 2 shows the structure of a transfer high-voltage control circuit in a conventional image forming apparatus. Known means for varying a voltage applied to the transfer roller 5 is control that uses a pulse width modulation (PWM) system (refer to, for example, Japanese Patent No. 2951993). A PWM signal outputted by a high-voltage control section 31 passes through an LPF (Low Pass Filter) 33 provided on a primary side of a high-voltage transformer 32. The signal is thus converted into an analog signal of 0 to 5 V and has its voltage changed to become a transfer bias. Specifically, PWM control is provided to modulate the duty ratio of the pulse signal to change the voltage behind the LPF 33. The generated voltage changes in proportion to the above change. For example, if the high-voltage transformer 32 has a maximum output voltage of 5 kV, when the PWM duty ratio is 100%, 5 kV is outputted. When the PWM duty ratio has a resolution of 256 bits, a voltage per bit is about 20 V. This resolution is sufficient for the transfer high voltage. The high resolution is characteristic of the PWM system.
Another transfer voltage control system is an ATVC (Active Transfer Voltage Control) system (refer to, for example, Japanese Patent No. 2614309). The ATVC system carries out constant voltage control during a non-paper-passing period when no print materials are present in the transfer section. The ATVC system then determines a constant voltage control value for a paper passing period on the basis of a currently retained voltage. A transfer application bias is determined on the basis of 1) a multiple of the retained voltage, 2) a multiplication of the retained voltage by a coefficient, 3) a constant voltage, or 4) a combination of 1) to 3), using appropriate timings in a particular sequence. With the PWM method, a PWM value occurring during constant voltage control is retained. Then, on the basis of this PWM value, a PWM value is determined which is used for constant voltage control while paper is being passed through the apparatus.
A description will be given of an example of a transfer control sequence for the conventional image forming apparatus. Constant current control is started at a predetermined time during forward rotation after a print signal has been received. A PWM value that corresponds to a desired current value is stored, and a PWM average value for one rotation of the transfer roller is defined as PWMo (a high-voltage output value corresponding to PWMo is defined as Vo). After the constant current control has been finished and before a leading end of the print material reaches the transfer nip inlet Na, the transfer bias control value remains at PWMo (constant voltage Vo control). Subsequently, while a toner image is being transferred, PWM based on the PWMo, that is, a print bias: PWMt=a*PWMo+b (a and b are constants; PWMt>PWMo) is outputted (the high voltage output value corresponding to PWMt is defined as Vt). Then, before a trailing end of the print material reaches the transfer nip inlet Na, the PWMt output is switched to the PWMo output. Subsequently, the application of the transfer high voltage is turned off at a predetermined time to complete transfer control.
Control is conventionally used in which the transfer bias is switched from Vt to Vo when the trailing end of the print material passes through the transfer section and is reduced when there are no print materials in the transfer section N (this control will hereinafter be referred to as non-paper-passing bias control). This control prevents the surface of the photosensitive member from being disadvantageously charged with a positive bias voltage received from the transfer roller while no paper is being passed (hereinafter referred to as photosensitive member plus memory). For example, as disclosed in Japanese Patent Application Laid-open No. 2001-083812, a switch timing is ordinarily set at a time when a bottom margin of an image is transferred, that is, a time after a toner image has been completely transferred and before the trailing end of the print material reaches the transfer nip N. Furthermore, while the trailing end of the print material is passing through the transfer nip N, the photosensitive member plus memory produces notably significant effects. Accordingly, while the trailing end of the print material is passing through the transfer nip N, weak bias application control (hereinafter referred to as trailing end bias control) having a transfer bias control value of Vo/2 or less may be provided.
However, in the conventional image forming apparatus, inappropriate conveyance or jamming may be caused by the following factors in connection with the passage of print materials such as index cards used in a low humidity environment which materials have a certain level of rigidity and which is light and small.
A dry high-resistance print material is likely to retain charges on its nonprinted surface (the charges are unlikely to be attenuated). Accordingly, when the trailing end of the print material passes through the transfer nip outlet Nb, the plus charges retained on the nonprinted surface are likely to be attracted to the negatively charged surface of the photosensitive member via the trailing end of the print material. Consequently, the state shown in FIG. 1 is likely to result.
For example, a small-sized sheet of a high resistance which has been left and dried in a low humidity environment, for example, an index card P of 3×5 inch size (76.2 mm×127 mm) and 0.3 mm thickness, is passed through an image forming apparatus having a distance of 200 mm between the transfer roller and the fixing device. After the trailing end of the index card P in the conveying direction has come out of the transfer section and before its leading end reaches the fixing section, the index card P floats owing to its rigidity and light weight so that its trailing end extends along a rotating direction a of the photosensitive drum 1. The index card P cannot contact with the conveying belt 16a and remains between the transfer roller and the fixing device. As a result, jamming may result.
In particular, in an image forming apparatus which has a print speed of more than 100 mm/sec and in which the high voltage transformer has a fall time (in this case, the time from the switching from the transfer bias Vt to trailing end bias control until a drop to half of Vt or less) of 0.05 sec or more, the trailing end of the print material passes through the transfer nip N early during a fall time. Consequently, the bias drop cannot appropriately follow the passage and an excessive amount of charges are likely to be retained at the trailing end. Therefore, the above-described jamming may result.
The transfer bias control in the conventional image forming apparatus will be described with reference to FIGS. 3A and 3B. FIG. 3A shows a transfer bias control value for the vicinity of the trailing end of the print material in an image forming apparatus having a print speed of 150 mm/sec, a transfer high-voltage transformer fall time of 0.05 sec, and a transfer nip width of 4 mm. FIG. 3B shows the effective value of the transfer bias. The time at which the trailing end of the print material P passes through the central position No of the transfer nip is defined as a time reference zero. The switching timing for the trailing end bias control is −17 msec, when a position of the print material P located 2.5 mm away from its trailing end passes through the nip center No. The trailing end bias control has a bias value of Vo/2. The switching timing for the non-paper-passing bias control is +17 msec. The non-paper-passing bias value is Vo.
FIG. 3B indicates that while the trailing end of the print material is passing through the transfer nip N, the trailing end bias control applies Vo/2. However, a high voltage of Vo or more is actually applied under the effects of the print speed and fall time. If the bias switching timing corresponds to the time immediately before the trailing end of the print material passes through the transfer section, substantially no trailing end bias acts on the trailing end of the print material owing to the tradeoff between the print speed and high-voltage fall time. As a result, the apparatus may be jammed with the above described small-sized paper.